BaseVarDumper::export() added
Doc comments for `BaseVarDumper::export()` added
Issue #3232 added to CHANGELOG
Object handling at `BaseVarDumper::export()` reworked
`VarDumper::exportAsString()` removed
Merge branch 'master' of github.com:yiisoft/yii2 into 3232-var-export
Merge pull request #3419 from klimov-paul/3232-var-export

Fix for #3232 var export